For AIIMS the minimum qualifying percentage for SC candidates is 40%.So there are200 questions in total in AIIMS.Each question carries 1 marks for each positive response and 0.33 marks is deducted for each wrong answer.Thus the total marks is 200.

40% of 200 is 80.So you have to attempt minimum 80 correct questions without any negatives to qualify the exam.Its safe to attempt 100-120 questions atleast considering the possibility of negatives.

Hope this helps. 

Thank you.